A thin film is generally a layer of material ranging from fractions of a nanometer to several microns in thickness. Crystalline silicon solar cells in particular may benefit from thin film construction. For example, a silicon-containing film may be used to enhance a dopant profile or to form a junction on a silicon substrate. More detail may found in U.S. patent application Ser. No. 12/656,710, entitled Methods Of Forming A Multi-Doped Junction With Silicon-Containing Particles filed on Feb. 12, 2010 and incorporated by reference.
In general, a densified silicon containing particle film, particularly a nanoparticle film, has substantially more surface area than the underlying substrate. Nanoparticles are generally microscopic particles with at least one dimension less than 100 nm. A silicon-containing nanoparticle film may have over 10,000 times the surface area of the underlying substrate.
Nanoparticles may be produced by a variety of techniques such as evaporation (S. Ijima, Jap. J Appl. Phys. 26, 357 (1987)), gas phase pyrolysis (K. A Littau, P. J. Szajowski, A. J. Muller, A. R. Kortan, L. E. Brus, J Phys. Chem. 97, 1224 (1993)), gas phase photolysis (J. M. Jasinski and F. K. LeGoues, Chem. Mater. 3, 989 (1991)), electrochemical etching (V. Petrova-Koch et al., Appl. Phys. Lett. 61, 943 (1992)), plasma decomposition of silanes and polysilanes (H. Takagi et al, Appl. Phys. Lett. 56, 2379 (1990)), high pressure liquid phase reduction-oxidation reaction (J. R. Heath, Science 258, 1131 (1992)), etc.
Etchants are often used to control the final thickness of a deposited silicon-containing film. For silicon-containing films (and for the underlying crystalline silicon substrates) etching generally involves breaking of atomic silicon bonds at defects in the crystal structure, such as at surface regions and grain boundaries (in the case of multi-crystalline solar cells).
However, because both the silicon-containing film and the underlying substrate contain silicon, common silicon etchant techniques such as KOH and fluorine-based etchants containing oxidizers are generally not selective and thus may also detrimentally etch the underlying substrate.
Fluorine-based etchants usually contain a source of fluoride ion F− to remove SiO2 (hydrofluoric acid HF and/or its ammonium or sodium salts (NH4F, NaF, NaHF2)) and a strong oxidizer to convert Si into its oxide (nitric acid HNO3, hydrogen peroxide H2O2, potassium permanganate KMgO4, potassium chromate K2CrO4, iodine I2, etc.). As a result of etching, roughness of surfaces increases, which may be desired for example, to increase light trapping in solar cells manufacturing. In addition, such etchants also tend to leave residual oxide due to the strong oxidizer presence, which may be problematic and may need to be removed with an additional process step, such as with diluted HF in order to achieve hydrogen-passivated hydrophobic surfaces needed for subsequent solar cell processing steps.
However, exclusion of oxidizer from the etching mixture prevents efficient etching of Si. For example, treatment of Si(111) surfaces with buffered NH4F not containing oxidizer, results in atomically flat H-passivated Si(111) surfaces without any substantial silicon etching (Christopher P. Wade and Christopher E. D. Chidsey: Appl. Phys. Lett. 71 (12), 22 Sep. 1997, pp. 1679-1681; and Sang-Eun Bae, Mi-Kyung Oh, Nam-Ki Min, Se-Hwan Paek, Suk-In Hong, and Chi-Woo J. Lee: Bull. Korean Chem. Soc. 2004, Vol. 25, No. 12, pp. 1822-1828).
In view of the foregoing, there is a desire to provide selective methods of etching silicon-containing films on silicon substrates without residual oxidation.